Dump leaching is now an accepted method of recovering copper from sub-standard copper-bearing materials including some material categorized as waste, such as dumps formed in earlier decades from copper mining operations.
Most leach dump operations involve a mixed oxide-sulfide ore and are leached with dilute sulfuric acid solutions. Generally, dump leaching is performed with minimal solution control and recoveries do not exceed 40 to 60% of the copper present in the ore after periods up to five years of leaching. Recoveries are even poorer when the ore body to be leached contains totally or substantially totally oxidized copper minerals such as tenorite and melaconite. In such ore bodies, a large portion of the copper, up to 50% or more, assays as non-acid soluble copper which, in essence, lowers the effective grade of the material to be leached by 50%. Thus, with an ore body having about 0.3% copper, the effective grade of the material to be leached is only 0.15%.
Efforts to improve upon the recovery of the non-acid soluble fraction have not been successful. Such efforts have included the use of high concentrations of sulfuric acid to leach, over 100 g/l, and techniques involving crushing the ore, agglomerating with a strong acid, and curing for a period of time before performing the typical leaching. In the case of leaching with high concentration of acid, there were the evident problems of high acid consumption by the ore and high residual acid off the dumps to the precipitation plant. The high cost involved in terms of cost of acid and handling of the high residual acid make such procedure uneconomical and undesirable. At today's high cost, crushing and handling of the ore is also unsuitable because it is uneconomical.